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Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing
leucitites characterized by high levels of incompatible trace elements with an
orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U,
and Pb. Initial values of
87
Sr/
86
Sr range from 0.70926 to 0.70999,
143
Nd/
144
Nd
ranges from 0.51213 to 0.51217, while the lead isotope rations vary between
18.788 and 18.851 for
206
Pb/
204
Pb, 15.685
207
Pb/
204
Pb,
15.701 for
and
-
39.076 for
208
Pb/
204
Pb. They observed that shoshonites show a similar
pattern of trace element depletions and enrichments to the earlier ultrapotassic
leucite-bearing rocks but have a larger degree of differentiation and lower con-
centrations of incompatible trace elements. On the other hand, shoshonitic rocks
have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic
leucite-bearing rocks. The
87
Sr/
86
Sr ratio ranges from 0.70665 to 0.70745,
143
Nd/
144
Nd varies from 0.51234 to 0.51238,
206
Pb/
204
Pb ranges from 18.924 to
19.153,
207
Pb/
204
Pb varies from 15.661 to 15.694, and
208
Pb/
204
Pb ranges from
39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values
between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of
incompatible trace element contents. They have argued that ultrapotassic magmas
were generated in a modi
39.048
-
ed lithospheric mantle after crustal-derived metasoma-
tism. Interaction between the metasomatic agent and lithospheric upper mantle
produced a low-melting point metasomatized veined network. They think that
partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic
magmas. It was possibly triggered by either post-collisional isotherms relaxation or
increasing temperature due to increasing heat
flow through slab tears. Shoshonitic
magmas were generated by further melting, at higher temperature, of the same
metasomatic assemblage with addition 10
20 % of OIB-like astenospheric mantle
material. They suggest that addition of astenospheric upper mantle material from
foreland mantle,
-
flowing through slab tearing after collision was achieved.
4.8.4.8 Somma-Vesuvius
The Somma-Vesuvius complex (Cundari and Sulviulo 1987) is marked by the
presence of the famous active volcano Vesuvius, which is half encircled by the
breached caldera called Monte Somma (Fig.
4.23
). The
first eruption of the volcano
took place in 79 A.D., followed by a long period of quiescence. It erupted again in
1,631, and then after a gap of 130 years it erupted in 1879. The last two eruptions
were observed in 1906 and 1944 (Fig.
4.24
).
The Somma-Vesuvius volcanic complex may be considered as a classic locality
for leucite-bearing assemblages. Rittmann (1933) described evolution of Vesbian
complex and considered that temporally the trachytes were the earliest, followed by
Somma-Vesuvius suite of leucite tephrites, tephritic leucitites (earlier termed as
vesbites) and their phonolitic variants.
The trachytic rocks underlie the pyroclastic deposits, which erupted from the
adjoining Phlegrean complex, preceded by eruptive cycles of the Somma activity.
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